Aminoacyl-tRNA synthetases are essential for the correct pairing of amino acids and tRNAs during protein synthesis. Most aminoacyl-tRNA synthetases belong to one of two unrelated structural classes. The only widespread exceptions are the lysyl-tRNA synthetases, which are class I enzymes in certain bacteria and archaea but are otherwise members of class I1. The class I lysyl-tRNA synthetase is found in a number of pathogenic bacteria and is fundamentally different from the human class II enzyme, identifying the class I bacterial enzyme as a potential target for developing anti-infective agents. The aim of this proposal is to investigate structure/function relationships in the recently discovered class I-type lysyl-tRNA synthetases. The major goals include: i) Determining how class I and class II lysyl-tRNA synthetases recognize and discriminate between lysine and lysine analogues. ii) Identifying the protein-RNA interactions that determine lysine tRNA recognition in vitro and in vivo. The results of these experiments will reveal how the same activity (attachment of lysine to lysine tRNA) can be achieved within two completely different structural frameworks. This, in turn, will provide a framework for developing therapeutics specifically targeted against the class I lysyl-tRNA synthetase.